The purpose of this study was to measure the validity and reliability of a portable telemetry system (Cosmed K2) for measurement of oxygen consumption (V̇O2) during a laboratory exercise test at submaximum and maximum intensities. Twenty subjects performed three testing sessions on a treadmill on three consecutive days. Each testing session consisted of a submaximal and a maximal exercise test. On one day, V̇O2 was determined with the conventional Douglas bag method, and on the other two days, V̇O2 was determined using the Cosmed K2 system. Utilization of the Douglas bag method or the Cosmed K2 was randomly assigned. The results indicated that the Cosmed K2 instrument is reliable and valid (p>0.05) for measurement of V̇O2 (1 · min-1) during laboratory exercise testing at submaximum and maximum intensities. The assumption made by the K2 apparatus of a constant respiratory gas exchange ratio (R) of 1.00 did not have a significant influence on V̇O2 measurements.